6. Demonstration House
6.2 Measurements of the Old Heating System
6.2.2 Energy Balance of the Old Heating System
In the demonstration house the old heating system based on a non condensing natural gas boiler was measured for a period of 21 months from August 2004 till April 2006.
The monthly energy balance for this system is presented in Fig. 6–15 and Table 6–1.
The definitions of the energy values and how they are measured are described in chapter 6.2.1 and Fig. 6–13. First of all, two points need to be explained in order to understand the graphs and tables:
• Conversion of measured cubic meter natural gas to energy:
To convert the natural gas consumption from volume to energy, the following heating value was used:
Low heating value: 10.67 kWh/m3 (= 11.02 kWh/mn3)
This is the average value of 42 measurement points in Denmark for the twelve months from April 2005 until March 2006. The maximum difference over these 12 months and all 42 measurement points was ± 0.5% of this chosen value. Due to this very small variation, it was decided to use one constant value for the complete evaluation in the old heating system.
It has to be noted, that all graphs and tables are based on the low heating value of the natural gas.
• The efficiencies which are presented are defined as (see Fig. 6–13, page 105):
1. Boiler Efficiency:
n Consumptio Gas
Natural
Heating Space
Heating Water
Hot Domestic
η_boil +
= Eq. 6–1
2. Natural Gas - COP:
n Consumptio Gas
Natural
Heating Space
n Consumptio Water
Hot Domestic
COP +
= Eq. 6–2
3. Domestic Hot Water Efficiency:
Heating Water
Hot Domestic
n Consumptio Water
Hot Domestic
η_DHW = Eq. 6–3
4. Hydraulic Efficiency:
Heating Space
Heating Water
Hot Domestic
Heating Space
n Consumptio Water
Hot Domestic d
η_hy
+
= + Eq. 6–4
“Boiler Efficiency” is the average efficiency of the boiler over a period and therefore including start/stop losses and standby losses.
“Natural Gas - COP” is the coefficient of performance based on the natural gas consumption that is calculated with the low heating value.
“Domestic Hot Water Efficiency” is not including the boiler efficiency, therefore it is an efficiency taking into account the heat losses of the hot water tank, the pipes between the hot water tank and the boiler and the hot water circulation losses.
Calculating the COPDHW only for hot water preparation is only possible for months when no space heating energy is measured:
n Consumptio Gas
Natural
n Consumptio Water
Hot Domestic boil
_ η
* DHW _ η
COPDHW = = Eq. 6–5
If this equation is used for months including space heating energy, it is important to have in mind that the boiler efficiency is an average value for the whole month and therefore strongly influenced by the boiler efficiency during space heating, which typically is much higher than the boiler efficiency during hot water preparation.
“Hydraulic Efficiency” is showing the overall system efficiency excluding the boiler efficiency. This key figure mainly will be used later to compare the new system with the old system without the influence of the different boilers.
As described in chapter 6.2, until 21/12-2004 the controller of the boiler was faulty and therefore the system was operating very inefficient. This can be observed in Fig.
6–15 especially from August until December 2004, where all three efficiencies are dramatically low. Also the energy demand for “Domestic Hot Water-Heating” is abnormal high compared to “Domestic Hot Water-Consumption” (about 2.5 times
higher). After the boiler was repaired on 21/12-2004, the results are much better. On 21/12-2004 the three main problems were solved:
1. The internal heat exchanger was exchanged because the old one was leaking.
2. The controller was repaired in such a way that if the burner was switched off, also the internal pump and the combustion air ventilator was switched off.
3. The controller also was repaired in such a way that during space heating the forward temperature was not the same as for hot water preparation, which was 85°C.
Due to the fact that the ambient temperature sensor had no contact to the controller, the space heating forward temperature still was not controlled according to the set heating curve. The effect of this will be explained later in chapter 6.2.3.
The summer months July and August 2005 show the very typical decrease of the efficiencies if (almost) only hot water preparation demand has to be supplied.
On 21/11-2005 the hot water circulation pump was set in operation by the electrician.
The pump was controlled by a clock and was in operation daily from 06:00 to 10:00 and 17:00 to 20:00. On 20/12-2005 the house owner decided to switch off the hot water circulation pump because of the huge heat losses. As the numbers in Table 6–1 show, the heat losses of 297 kWh during 30 days are in same order as the hot water consumption. In other words, the hot water circulation losses were about 100% of the hot water consumption, and this just based on 7 operating hours per day. The reasons for this high circulation loss are explained in the next chapter more detailed.
Looking on the tendency of “Domestic Hot Water Efficiency” from January 2005 until April 2006 it can be observed that this efficiency is decreasing from around 80%
to less than 70%. This tendency fits to the also occurring changes of the hot water consumption in the same period. In the first three months in 2005 the consumption in average was about 11 kWh/day compared to about 9 kWh/day in the first three months in 2006. Further, due to any reasons also the temperature difference of the hot water consumption increased by about 5 K (from around 56 to 61°C). Since the cold water temperature evolution typically is very similar from year to year it can be assumed that the set point temperature of the hot water tank was higher. In December 2005 again (after December 2004) the internal heat exchanger of the natural gas boiler had to be replaced due to leakage and maybe at that time something changed the setting. This of course is leading to higher standby losses of the hot water tank and the pipes between the boiler and the hot water tank. Since the hot water mixing valve at hot water tank outlet did not operate properly (see later in Fig. 6–17: hot water temperature up to 70°C) the heat loss effect of hot water staying in the pipe after a tapping is increased also quite strong.
Therefore this is assumed to be the explanation for the quite strong reduced
“Domestic Hot Water Efficiency” within this period.
In the months May, June and September, October 2005 the boiler efficiency is about 5 per cent points higher than in the core heating period. This is most likely due to the lower temperature level of the space heating system and the higher combustion air temperature during this period which both is reducing the exhaust gas losses. Again most likely this is also influenced by the fact that the ambient temperature sensor was not connected to the boiler controller.
REBUS project Demonstration House - HELSINGE Energy Balance - 08-2004 till 04-2006 - Monthly Values
0 400 800 1200 1600 2000 2400 2800 3200 3600 4000 4400 4800
Aug-04 Sep-04 Oct-04 Nov-04 Dec-04 Jan-05 Feb-05 Mar-05 Apr-05 May-05 Jun-05 Jul-05 Aug-05 Sep-05 Oct-05 Nov-05 Dec-05 Jan-06 Feb-06 Mar-06 Apr-06
Energy [kWh]
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
110%
120%
Efficiency [%]
Natural Gas Consumption [kWh] Space Heating [kWh] Domestic Hot Water-Heating [kWh]
Domestic Hot Water-Consumption [kWh] Electricity [kWh] Domestic Hot Water Efficiency [%]
Boiler Efficiency [%] Natural Gas - COP [%] Hydraulic Efficiency [%]
Fig. 6–15 Energy balance for the old heating system in the demonstration house.
Table 6–1 Energy data for the old heating system in the demonstration house
Ambient Temperature Average Source: DMI Natural Gas Consumption Low Heating Value = 10.67 kWh/m3 Space Heating Space Heating Space Heating Temperature Difference Monthly average Domestic Hot Water-Heating Domestic Hot Water-Consumption Domestic Hot Water-Consumption Average daily consumption Domestic Hot Water Consumption Temperature Difference Monthly average Domestic Hot Water-Circulation (21/11-05 till 20/12-05; daily 6-10 and 17-20) Electricity Heating System (from 21/11-05) Boiler Efficiency (Boiler/Gas) Domestic Hot Water Efficiency Natural Gas - COP (DHW+SH)/(Gas) Circulation as Loss Hydraulic Efficiency (DHW+SH)/(Gas*eta_boil) Circulation as Loss
Gas SH DHW Circ eta_boil
[°C] [kWh] [kWh] [m3] [K] [kWh] [kWh] [kWh/d] [K] [kWh] [kWh] [%] [%] [%] [%]
08-2004 17.7 507 14 1.1 10.9 274 99 3.2 58.1 0 - 56.8% 36.1% 22.3% 39.2%
09-2004 13.5 460 33 1.7 17.0 241 98 3.3 60.3 0 - 59.6% 40.7% 28.6% 47.9%
10-2004 9.4 2588 1292 30.7 36.3 773 273 8.8 60.3 0 - 79.8% 35.3% 60.5% 75.8%
11-2004 4.7 3382 1943 43.8 38.2 844 319 10.6 62.5 0 - 82.4% 37.8% 66.9% 81.2%
12-2004 3.3 3382 2090 120.6 14.9 773 348 11.2 60.5 0 - 84.6% 45.0% 72.1% 85.2%
01-2005 2.6 3055 2322 243.8 8.2 391 315 10.2 55.8 0 - 88.8% 80.5% 86.3% 97.2%
02-2005 -0.3 3477 2665 223.8 10.3 434 350 12.5 57.1 0 - 89.1% 80.6% 86.7% 97.3%
03-2005 0.9 3602 2822 234.0 10.4 418 324 10.5 57.0 0 - 89.9% 77.5% 87.3% 97.1%
04-2005 7.8 2224 1685 218.8 6.6 373 292 9.7 54.6 0 - 92.5% 78.3% 88.9% 96.1%
05-2005 11.0 1655 1207 182.1 5.7 342 257 8.3 52.6 0 - 93.6% 75.2% 88.5% 94.5%
06-2005 14.1 1038 663 90.2 6.3 304 229 7.6 50.6 0 - 93.1% 75.2% 85.9% 92.2%
07-2005 17.2 400 15 0.9 13.8 320 220 7.1 48.1 0 - 83.8% 68.7% 58.8% 70.1%
08-2005 15.2 401 77 16.5 4.0 285 183 5.9 30.3 0 - 90.4% 64.2% 64.9% 71.8%
09-2005 13.9 808 424 105.9 3.4 340 241 8.0 54.5 0 - 94.5% 70.9% 82.3% 87.0%
10-2005 10.3 1890 1378 227.1 5.2 391 281 9.1 57.0 0 - 93.6% 71.8% 87.8% 93.8%
11-2005 5.2 2926 2124 258.0 7.1 489 292 9.7 59.2 91 31 89.3% 59.7% 82.6% 92.5%
12-2005 1.9 3654 2593 218.5 10.2 644 295 9.5 59.4 206 98 88.6% 45.8% 79.0% 89.2%
2005 25131 17974 2019.6 7.7 4733 3279 9.0 52.8 - - 90.4% 69.3% 84.6% 93.6%
01-2006 -1.6 4639 3693 283.2 11.2 364 247 8.0 59.6 0 106 87.4% 67.8% 84.9% 97.1%
02-2006 -0.2 3871 2978 255.0 10.1 397 267 9.5 61.9 0 94 87.2% 67.2% 83.8% 96.1%
03-2006 -1.1 4257 3243 258.7 10.8 434 300 9.7 62.6 0 104 86.4% 69.1% 83.2% 96.4%
04-2006 5.9 2928 2210 273.2 7.0 398 259 8.6 61.5 0 98 89.1% 65.1% 84.3% 94.7%